drm/imx/dcss: fix resource size calculation

The resource is inclusive of the ->start and ->end addresses so this
calculation is not correct.  It should be "res->end - res->start + 1".
Use the resource_size() to do the calculation.

Fixes: 90393c9b ("drm/imx/dcss: request memory region")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Signed-off-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4914592b-4256-4c9c-bc1d-6dec1e473831@moroto.mountain